Clear Anodize as a Paint Base

Q. Our parts are generally Type III (hardcoat) anodized, sometimes duplex-sealed and sometimes unsealed. We just recently learned of the surfactant issue during sealing from one of your articles. Our end-products are subject to sea water, and we try to adhere paint and polyurethane potting compounds to some of the parts. Would paint adhesion or adhesive bonding be enhanced by abrasive blasting prior to anodizing, maybe at 30 to 60 grit? Some of our testing has shown that abrasive blasting may help, but we don’t know what effect it may have on the corrosion protection generally provided by anodizing. Our current plan is to clean, abrasive blast, clean, Type III anodize, duplex seal (no surfactants) and try to topcoat before getting contaminated. —B.T.

A. You are on the right path by eliminating surfactants in the sealing process. If Type II and Type III anodic coatings are sealed in this manner, it should provide an excellent base for the application of organic coatings (paint).

Bear in mind that anodic coatings produced on a "roughened" substrate (blasted, sanded, etc.) may be less corrosion-resistant than anodic coatings produced on a "smooth" substrate. While normally a roughened surface may provide more "tooth" for an adherent coating, the anodic coating can be rough and may not provide a high-quality, corrosion-resistant surface. Producing an anodic coating that is continuous and free of any voids is maximized when the substrate is smooth. An anodic coating produced on a "smooth" surface, when properly sealed, may provide enhanced corrosion resistance. Coating adhesion testing can be done on coupons that are rough and coupons that are smooth, and the results compared. It can then be determined if roughness of the substrate will be a significant factor. It sounds like you may have already done that. Corrosion testing of anodized, unpainted coupons with smooth and roughened surfaces may also be done, and the results compared.

It is best to use aluminum oxide media if you are going to blast the parts. Do not use glass beads (silica), as the silica can be imbedded in the aluminum substrate and can be difficult to remove in the cleaning process prior to anodizing. This may leave a gray or brownish haze on the parts after anodizing. Since the parts are painted, that may not be a concern. However, the coating quality may not be as good as it would be after using aluminum oxide as the blasting medium, because there could be silica particles at the substrate/anodic coating interface and/or trapped in the oxide itself. If you desire to blast with glass bead, I would recommend that the parts be etched after blasting in an acid etch bath containing fluoride in order to assure the removal of imbedded silica in the aluminum parts. This could be an ammonium bifluoride bath, or a mixture of nitric acid and hydrofluoric acid. Special precautions are required when using solutions that contain fluorides.